1. Field of the Invention
The present invention relates to a communications method and a communications system that can implement hard real-time communication required for various decentralized real-time control of robots, vehicles, plants, home automation etc., and soft real-time communication required for achieving smooth transmission of multimedia data like images and voices.
2. Description of the Related Art
In conventional real-time communication isochronous data transfers and the like) such as USB and IEEE 1394, a main communication control and management unit, which carries out centralized management of the bandwidth of communication data, is used to reserve the bandwidth of communication in advance and to perform exclusive management of the bandwidth. In other words, the main communication control and management unit enables the real-time communication by the centralized management of the bandwidth. In such a centralized management system, it is difficult to construct a large-scale real-time communications system (maximum of 127 nodes in the USB, and 63 nodes in the IEEE 1394). In addition, when the main communication control and management unit fails, all the communications are halted.
Furthermore, there are no practical conventional systems capable of assigning priority to communication packets to enable overtaking. Even an experimental system that allows overtaking of communication packets cannot control the real-time communication with enabling low latency overtaking of communication packets, with obviating the need for retransmission of communication packets, or with accelerating or decelerating communication packets by changing the priority at each communication node.
Japanese patent application laid-open No. 62-260452 (1987) implements a circuit switching by utilizing a multistage interconnection network (MIN) and by assigning priority to communication packets. It is originally a patent application for a telephone circuit that carries out packet or circuit switching by assigning priority to such requests as a called setup request, a calling setup request and an asynchronous data request. It implements soft real-time communication for voice packets by assigning higher priority to voice packets for communicating voice data, and realizes non-real-time data packet communication utilizing the remaining bandwidth. The system, however, inputs and outputs the communication packets through multistage subswitches depending on the number of ports, and this presents a problem of increasing the latency as the number of ports increases because of increasing the number of subswitch stages, although this can broaden the bandwidth. Furthermore, it is necessary for an initial communication to open a circuit, resulting in considerable latency. Moreover, a state can continue for a long time in which lower priority packets cannot be transmitted from a calling node because no circuit opens. Thus, although the system is suitable for the soft real-time communication like voice communication, it is unsuitable for hard real-time communication used for system control.
In the conventional systems, communication packets with the same network addresses are transmitted from a source node to a destination node through the same path at a certain instant, and hence it is impossible to establish one or more exclusive lines or one or more alternative paths for these communication packets at the same time.
Japanese patent application laid-open No. 58-151747 (1983) discloses a system that can assign prescribed classes to respective terminals connected to a packet switching system, and carry out routing in accordance with the classes (corresponding to priorities) assigned to the terminals. The classes are assigned to the terminals, and in a failure the packets from terminals assigned higher priority can pass through an alternative path using a routing table statically set in advance. The system can be set in such a manner that the packets from lower priority terminals are allowed to pass only through an alternative path in a normal state, and are discarded during a failure. However, because the classes are attached to the terminals, and the routing can be changed only in a transition from the normal state to a failure, dynamic change of the routing cannot be achieved in the normal state. In addition, since communication is carried out from a source node to a destination node only through a single path at a certain moment, it is impossible to perform simultaneous communication by establishing multiple paths in the routing.